LONDON -- How thin is too thin? Not even Kate Middleton has escaped the thorny debate.

The willowy princess-to-be's appearance and reported comments during a visit to Northern Ireland sparked some concerns Wednesday about whether she's losing too much weight ahead of the April 29 royal wedding. The subject even became a topic on Twitter.

In front-page newspaper photographs, Middleton appears to be in good health if slightly slimmer than usual, but reporters heard many in the Belfast crowd of well-wishers on Tuesday making comments that she looked a bit gaunt.

One woman, Heather Lindsay, told Middleton "not to lose any more weight," and said Middleton remarked that it was all part of the wedding plan.

Palace officials Wednesday refused to say whether Prince William's fiancee is trying to lose a few pounds before the big day. They also declined to specify her height, weight or to comment on her words in Northern Ireland.

"It was an off-the-cuff comment," said a press aide to Prince Charles who asked not to be identified because of palace protocol. "It's a hugely private matter."

Prince Charles' press aides have been protective of Middleton's privacy since the engagement was announced in November. Middleton was active in many sports during her university days and her weight hasn't seemed to have fluctuated much since then.

Even if Middleton is not dieting, it would not be uncommon or worrying for her to lose several pounds in the weeks before the wedding simply because of nerves, said Mark Niemierko, a top London wedding planner who has seen clients lose weight unexpectedly in the final days before the ceremony.

"Even those brides who don't plan to lose weight lose weight," he said. "This is the one day they will be the absolute center of attention, and these are pictures they'll have forever, so they want to be in good form. The same is true for the groom."

He said Middleton is naturally tall and skinny - like her sister, maid of honor Pippa Middleton - but does not seem dangerously slim.

"I think she looks perfect, but everyone has issues about their bodies and I'm sure she just wants to be very toned," he said.

On Twitter, a handful of people posted comments about Middleton's weight on Wednesday. One said she shouldn't lose any more weight and should serve as a healthy role model to young girls.

Nutritionists who deal with eating disorders - like the bulimia that plagued William's mother, the late Princess Diana - say it is usually fine for prospective brides to diet before their weddings as long as they don't try to lose too much weight, which can create an unhealthy "yo-yo effect" when the pounds are put back on before the honeymoon is over.

But nutritionist Amanda Hamilton said Middleton should be careful.

"She's already slim enough so I would have concerns over that," Hamilton said. "Anyone would look at her picture and ask why does she have to lose weight. She looks fantastic. She's on the very slim end of what would be considered healthy. She's not in the danger zone, I don't think ... but she doesn't have any weight to lose."